The present invention relates to a hearing aid, and more particularly, to a hearing aid having a programmable resistor which is programmed by a programming system external to the hearing aid.
Hearing aids having programmable resistances are known. Such resistors have a range of resistances and are part of circuits in the hearing aid which adjust input-output (I/O) characteristics of wide dynamic range compressors such as the compression ratio and the lower and upper values of sound pressure level (SPL) at which compressor action begins or ends (commonly referred to as the lower and upper threshold knee, respectively), which set the input-output characteristics of output-limiting compressors (typically the SPL at which limiting sets in), and which operate as tone controls to control the frequency response (most often the low frequency response with action similar to the treble and bass controls on a stereo set). Almost all hearing aids have adjustments for at least one of these characteristics, and more advanced hearing aids have need for four or more adjustments. In general, each of the adjustable characteristics are set for the wearer of the hearing aid by changing a resistance, usually accomplished by manually adjusting trimmers (rheostats) on the faceplate of the hearing aid. The faceplate becomes undesirably large as the number of electrically adjustable characteristics, and their attendant trimmers, is increased. In order to allow the hearing aid to have multiple electrically adjustable characteristics but to reduce or maintain the size of the faceplate, hearing aids with programmable resistances were developed. Before or during fitting of the hearing aid, an external programmer sends a control word through a port on the hearing aid to a set of programmable resistors on an integrated circuit within the hearing aid. One programmer employs three wires to program the programmable resistor to the desired resistance. The first and second wires supply a programming code and a clock signal to clock the programming code into the memory and the third wire supplies the programming voltage to the memory. Another programmer in development uses a single wire which permits several operations: 1) synchronize the clock in the programmer to the on-chip clock in the programmable resistor, 2) read the existing on-chip data stored in the on-chip non-volatile memory, 3) send new data to the chip where it is initially stored in volatile memory, and 4) provide the programming voltage (typically 10-20 volts at the present time) needed to transfer the new data to the non-volatile on-chip memory.
Each programmable resistor changes one of the electrically adjustable characteristics of the hearing aid. There are two general types of programmable resistors: one having a parallel network of resistor (FIG. 1A) and one having a series network of resistor (FIG. 1B). In either structure, a switch is combined with each resistor to either switch the resistor into the circuit so current flows through the resistor, or to shunt the current around the resistor so current flows through the switch instead of the resistor. In the case of the parallel network, the switch is in series with the resistor so that when the switch is activated, current flows through the switched-in resistor. In the series network, the switch is electrically in parallel with the resistor, so that when the switch is activated, current shunts around the resistor instead of flowing through the resistor. As several of these programmable resistors may be on the same integrated circuit, it is desirable to reduce the space that each programmable resistor occupies. Both the resistors and the switches contribute to the overall size of the programmable resistor. A resistor occupies a specific amount of space depending on the resistivity of material used and the length to width ratio of the resistor. The resistance of a resistor is given by: ##EQU1## The resistivity of a standard SiCr resistor material typically used in the hearing aid industry, is approximately 2000 ohms per square. On the other hand, switch size is a function of the resistance to be switched. In general, a switch should have an on-resistance of less than 20% of the resistance it switches into or out of the circuit. The on-resistance of a MOS switch is given by:
where K is a material constant, V.sub.GS and V.sub.T are the gate to source voltage and the threshold ##EQU2## voltage of the MOS device, respectively, and where L and W are the length and width of the channel of the MOS device. The size of the switch is a function of its length to width ratio.
Parallel resistor networks, such as the one shown in FIG. 1A, are well-suited for low valued programmable resistors, since the size contribution from the switches rather than the resistors dominates the overall size of the programmable resistor and because each resistor has a resistance necessarily larger than the overall programmable resistance. On the other hand, series resistor networks, such as the one shown in FIG. 1B, are especially well-suited for high valued programmable resistors, since the size contribution from the resistors dominates the contribution from the switches, and since all the resistors have a resistance necessarily less than the total programmable resistance.
A logarithmic programmable resistor, which is desirably used in hearing aids, has programmed resistances which progress in proportional relationship to log.sub.10 x. For example, a programmed minimum resistance of 1 k ohm might increase by a factor of 1.41 k ohm with each step in resistance, up to a maximum value of 100 k ohm or higher. For this example, the progression of programmable resistances would be 1 k ohm, 1.41 k ohm, 2.0 k ohm, 2.82 k ohm, 4.0 k ohm, etc. When the required accuracy of a logarithmically programmable resistor is high, many step sizes are required and large numbers of resistors and switches must be used to achieve the desired values. A control word which controls the switches must have as many bits as there are switches in the programmable resistor. For example, a 9 bit wide switch control word defines up to 2.sup.9 (512) various resistance values, while a 7 bit wide switch control word defines up to 2.sup.7 (128) various resistance values. It is easier to achieve a typical accuracy of 10% in each programmable resistance when there are 512 possible combinations of the resistances than when there are 128 possible combinations. In general, as the number of switches (and attendant resistors) decreases in a programmable resistor, the relative accuracy of the lower values of programmed resistance is compromised, since relative accuracy is an expressed percentage of the desired programmed resistance and because the number of low valued resistors is typically small (since the low valued resistors are undesirably large and require a companion large switch).
Furthermore, as each switch in either a series or a parallel programmable resistor network is controlled by a control signal, the complexity of the circuitry needed to provide the control signals to the switches is significant when the number of switches is large. Logarithmically programmable resistors typically have large numbers of low on resistance switches and large numbers of high resistance valued resistors, both of which factors contribute to a large physical size and the potential for excessive power consumption. Physical size is critical in that it limits the number of resistors which can be used in the smallest hearing aids and also affects the cost of the programmable resistor circuit. These programmable resistors also must necessarily provide a control signal to each of the switches, leading to undesirably complex control signal circuits.
There is thus a need for a hearing aid with a high accuracy programmable resistor programmed by a reduced number of control bits and having a wide range of logarithmically related programmed resistances, but which reduces the amount of integrated circuit area.